Exploration of Pluto

The exploration of Pluto began with the arrival of the New Horizons probe in July 2015, though proposals for such a mission had been studied for many decades.[1] There are no plans as yet for a follow-up mission, though follow-up concepts have been studied.[2][3]

Exploring Pluto was contemplated since its discovery by Clyde Tombaugh in 1930[citation needed], but Pluto presents significant challenges for exploration because of its small mass and great distance from Earth. The two probes of the Voyager program, launched in 1977 to explore Jupiter and Saturn, had also the ability for an extended mission to other targets. Pluto was rejected in favor of Titan, Uranus and Neptune.

One of many possibilities for the Voyager 1 spacecraft after its flyby of Saturn in 1980 was to use Saturn as a slingshot towards Pluto for a flyby as early as March 1986.[4][5] However, scientists decided that a flyby of Titan during the Saturn encounter would be a more important scientific objective. A subsequent flyby of Pluto was impossible, because the close approach of Titan meant it was also on a trajectory that slingshotted it upwards out of the ecliptic.[6] Because no mission to Pluto was planned by any space agency at the time, it would be left unexplored by interplanetary spacecraft for years to come.

Shortly after Voyager 2's flyby of Neptune and its findings at Triton in August 1989, scientists sought interest in a mission to Pluto and further studies for the existence of a Kuiper belt and Kuiper belt objects, potentially similar to Triton.[7] All four outer planets were explored by Voyager 2, launched in 1977, with close approaches to Jupiter, Saturn, Uranus, and Neptune. After a Neptune encounter, the alignment of Pluto made it impossible for Voyager 2 to continue there, leaving it on a trajectory out of the Solar System.

The two Voyager missions had the success criteria of just one of them reaching Saturn, which they far exceeded. After Voyager 2 successfully returned data from Neptune in 1989, planetary scientists looked to Pluto as the destination for a subsequent mission. In 1992, NASA's Jet Propulsion Laboratory (JPL) proposed the Pluto Fast Flyby mission. This became known as the Pluto Express, and eventually the Pluto-Kuiper Express. This project got delayed, and in 2000 the mission was cancelled, with NASA giving the reason of cost overruns.

The cancellation of Pluto Kuiper Express angered some of the space-exploration scientific community, which led to groups, such as The Planetary Society, lobbying NASA for either a reboot of the Pluto Kuiper Express or a restart of a mission to Pluto. Internal divisions within NASA, including its Scientific Advisory Council, also voiced support for a Pluto mission.[8] In response to the backlash caused by the cancellation of Pluto Kuiper Express, it was decided to inaugurate a new class of missions that would fit between the big-budget Flagship Program and the low-budget Discovery Program, creating a compromise for missions such as the former Pluto Kuiper Express, which proved to be too expensive for the Discovery Program. A competition was held, in which NASA would select a mission concept to fund as part of the first mission of the New Frontiers program.[9]

One of the many early concepts for a mission to Pluto was to send a Mariner Mark II spacecraft. This was later ruled out in favor of a smaller, less expensive spacecraft similar to the "Pluto 350" concept.

In May 1989, a group of scientists and engineers, including Alan Stern and Fran Bagenal, formed an alliance called the "Pluto Underground". It was named in homage of the Mars Underground, another group of scientists that successfully lobbied for the restart of missions to Mars, following the lack of such since the Viking program. The group started a letter writing campaign which aimed to bring to attention Pluto as a viable target for exploration.[10] In 1990, because of pressure from the scientific community, including those of the Pluto Underground, engineers at NASA decided to look into concepts for a mission to Pluto. At the time, it was thought that the atmosphere of Pluto would freeze and fall to the surface during winter, and so a lightweight spacecraft was desirable, as it would be able to reach Pluto before such an event would occur. One of the earliest concepts was for a 40-kilogram spacecraft that would reach Pluto in five to six years. The idea was shortly scrapped, however, because of the infeasibility of miniaturizing scientific instruments aboard such a spacecraft to that size.[8]

A diagram of Pluto 350.

An artist's impression of Pluto 350

Another mission concept, known as Pluto 350, was developed by Robert Farquhar of the Goddard Space Flight Center, with Alan Stern and Fran Bagenal of the Pluto Underground, who both served as study scientists for the project. Pluto 350 aimed to send a spacecraft, weighing 350 kilograms, to Pluto.[7] The spacecraft's minimalistic design was to allow it to travel faster and be more cost-effective, in contrast to most other big-budget projects NASA were developing at the time, such as Galileo and Cassini. Pluto 350, however, would later become controversial among mission planners at NASA, who considered the project to be too small and too high-risk. An alternative plan which was considered at one point was to send to Pluto a configuration of the Mariner Mark II spacecraft, which would weigh 2,000 kilograms and cost US$3.2 billion, in sharp contrast to Pluto 350's $543 million cost.[8] While both projects competed for approval, Pluto 350 was more favored by NASA mission planners, who were starting to adopt smaller missions such as Mars Pathfinder and NEAR Shoemaker.[7]

For many years, people had waved that stamp around as sort of a call to arms, as a motivating graphic: "Not yet explored". That stamp had been in so many presentations by that point, I knew it would please people to have it go along.

In October 1991, the United States Postal Service released a series of stamps commemorating NASA's exploration of the Solar System.[11] The series featured a stamp for all planets, displaying an image of the planet and highlighting an associated spacecraft which was sent to it. The stamp for Pluto, however, depicted a featureless sphere, presented with the phrase "not yet explored" in place of the name of a spacecraft. The stamps were unveiled in a ceremony at the Jet Propulsion Laboratory. Two scientists who attended the event, World Space Foundation president Robert Staehle and JPL scientist Stacy Weinstein, were inspired by Pluto's status on the stamp, such that they started to inquire about the feasibility of sending a spacecraft to Pluto.[12][13] Engineers at the Jet Propulsion Laboratory, inspired by the "Not Yet Explored" status of Pluto, also started to put forward ideas about a mission to Pluto.[14][15]

In August 1992, Staehle telephoned Pluto's discoverer, Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it", Tombaugh later remembered, "though he's got to go one long, cold trip".[16] That year, Staehle, with the help of JPL engineers and students from the California Institute of Technology, formed the Pluto Fast Flyby project. The mission heralded the same ideology as the Pluto 350 concept: small in size and cost-effective in scope, so that the spacecraft would be able to get to Pluto faster and be affordable to develop and launch. Described as a "radical" mission concept, the mission would see two spacecraft being sent to Pluto. Both spacecraft were to weigh only around 35-50 kilograms each (including 7 kg worth of scientific instruments), and the project would cost less than US$500 million to develop, excluding launch costs.[12] Described by Staehle as a "faster, better, [and] cheaper" approach than the Pluto 350 and Mariner Mark II projects, it caught the attention of then-NASA Administrator Daniel S. Goldin, who ordered all work on both Pluto 350 and Mariner Mark II to cease and shift all resources to the new Pluto Fast Flyby project instead.[7]

During the development of Pluto Fast Flyby, however, there were multiple concerns from both NASA, Administrator Goldin and the mission's development team. As research and development into the mission progressed, the project's size, scope, and budget all expanded. Additionally, morale among the team and personnel working on interplanetary missions was low following the loss of the Mars Observer spacecraft during its attempted Areocentric orbit insertion in August 1993. Alan Stern would later cite that event as a significant factor towards the low enthusiasm for the Pluto Fast Flyby project.[8] The spacecraft were intended to be launched using Titan IV rockets, which would have cost US$400 million each, thus raising the budget to over US$1 billion.[7] Because of growing budget constraints, the dual-spacecraft concept was scrapped in favor of sending a single spacecraft to Pluto. The project was still too expensive, however, in the eyes of Administrator Goldin.[8] Alan Stern, as a compromise, reached an agreement with Russian Space Research Institute scientists in Moscow, in which Pluto Fast Flyby would be launched atop a Proton rocket, saving NASA over US$400 million in launch costs. Alec Galeev, head of the Russian Space Research Institute, reached the agreement with Stern by stipulating that Russia would include an atmospheric probe that would impact Pluto after studying its atmosphere with a mass spectrometer.[17] The proposal was forwarded to Administrator Goldin, but he vetoed the proposal, recommending instead that the JPL look into the feasibility of Pluto Fast Flyby being launched aboard a smaller rocket, such as the Delta II.[7]

Concept art for the Pluto Kuiper Express; the last iteration of the original Pluto mission concepts, eventually cancelled in 2000.

During the course of the late 1990s, a number of Trans-Neptunian objects were discovered, confirming the existence of a Kuiper belt. Interest in a mission to the Kuiper belt arose such that NASA instructed the JPL to re-purpose the mission as not only a Pluto flyby, but also a Kuiper belt object (KBO) flyby. The mission was thus re-branded as the Pluto Kuiper Express, after briefly being billed as Pluto Express prior to the revision. The weight of the spacecraft was raised again, this time to 175 kilograms, and NASA allowed further liberty with the project's budget.[7]

However, Goldin later decided that Pluto Kuiper Express was of low importance, and thus cut funding to the project drastically. Eventually, despite official selection of scientific instruments and the appointment of several investigators, then-Science Mission DirectorateEdward J. Weiler ordered the cancellation of the entire Pluto and Kuiper belt mission in 2000, citing growing budget constraints, which had plagued the project since its inception in 1992. At the time of cancellation, the projected costs surpassed $1 billion.[8][9]

A Pluto orbiter/lander/sample return mission was proposed in 2003. The plan included a twelve-year trip from Earth to Pluto, mapping from orbit, multiple landings, a warm water probe, and possible in situ propellant production for another twelve-year trip back to Earth with samples. Power and propulsion would come from the bimodal MITEE nuclear reactor system.[18]

After an intense political battle, a revised mission to Pluto called New Horizons was granted funding from the US government in 2003.[19]New Horizons was launched successfully on 19 January 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.[20]

First Pluto sighting from New Horizons in 2006

New Horizons captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager.[21] The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a gravity assist from Jupiter.

On 4 February 2015, NASA released new images of Pluto (taken on 25 and 27 January) from the approaching probe.[22]New Horizons was more than 203,000,000 km (126,000,000 mi) away from Pluto when it began taking the photos, which showed Pluto and its largest moon, Charon. On 20 March 2015, NASA invited the general public to suggest names for surface features that will be discovered on Pluto and Charon.[23] On 15 April 2015, Pluto was imaged showing a possible polar cap.[24] Between April and June 2015, New Horizons began returning images of Pluto that exceeded the quality that the Hubble Space Telescope could produce.[25][26]

Pluto's small moons, discovered shortly before and after the probe's launch, were considered to be potentially hazardous, as debris from collisions between them and other Kuiper belt objects could have produced a tenuous dusty ring. If New Horizons had travelled through such a ring system, there would have been an increased risk of potentially disabling micrometeoroid damage.[27]

New Horizons had its closest approach to Pluto on 14 July 2015—after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. New Horizons used a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyze Pluto's neutral atmosphere and its escape rate. New Horizons also photographed the surfaces of Pluto and Charon.

Photographs of Pluto taken on 14 July 2015 taken 15 minutes after New Horizon's closest approach, from a distance of 18,000 kilometers and sent to Earth on 13 September 2015 show a near-sunset on Pluto with details of the surface and a haze in the atmosphere.[28][29]

No follow-up missions to New Horizons have been formally planned, but at least two mission concepts have been studied. In April 2017, a workshop met in Houston, Texas to discuss ideas for a follow-up mission.[30] Possible objectives discussed by the group for a follow-up mission include mapping the surface at 30 feet per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, and topographic mapping of Pluto's regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using infrared laser pulses. According to New Horizons principal investigator Alan Stern, “If we send an orbiter, we can map 100 percent of the planet, even terrains that are in total shadow." [30] Stern and David Grinspoon have also suggested that an orbiter mission could search for evidence of the subsurface ocean hinted at in New Horizons data.[31]

Artist's impression of the Fusion-Enabled Pluto Orbiter and Lander at Pluto, with a New Horizions image of Pluto in the background.

Shortly after the New Horizons flyby, Stern suggested a Charon lander as a follow-up that would observe Pluto from Charon's surface.[32] However, such a lander would only observe the Charon-facing hemisphere of Pluto, as Pluto and Charon are tidally locked. Since the Houston workshop, Stern changed his mind to advocate instead for a Cassini-style orbiter that would use Charon's gravity to adjust its orbit while studying Pluto and its moons.[32] The probe could use electric propulsion similar to NASA's Dawn mission. It would then have the option of using Charon's gravity to leave the Pluto system after all Pluto science objectives are completed and study new KBOs beyond Pluto. Stern envisaged the probe being launched in 2030, marking the 100th anniversary of Pluto's discovery, and spending 7-8 years traveling to the Pluto system.

The Fusion-Enabled Pluto Orbiter and Lander was a 2017 phase I report funded by the NASA Innovative Advanced Concepts (NIAC) program.[3][33] The report, written by principal investigator Stephanie Thomas of Princeton Satellite Systems, Inc., describes a Direct Fusion Drive (DFD) mission to Pluto. A fusion reactor would be used to send a 1000 kg orbiter and lander to the Pluto system in only four years (more than twice as fast as New Horizons).

Global Aerospace Corporation presented a Pluto lander concept titled "Pluto Hop, Skip, and Jump" at the 2017 NIAC Symposium in Denver, CO.[34][2] The concept describes an entrycraft that would brake using the drag of Pluto's thin but highly spread-out atmosphere. Once on Pluto's surface, the vehicle would exploit Pluto's low gravity to hop between sites using propellant. This is similar to NASA's Triton Hopper concept for exploring Neptune's largest moon Triton.

1.
Pluto
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered, Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its planethood was questioned following the discovery of objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered and this led the International Astronomical Union to define the term planet formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet, Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has an eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five moons, Charon, Styx, Nix, Kerberos. Pluto and Charon are sometimes considered a system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, on July 14,2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto, on October 25,2016, at 05,48 pm ET, the last bit of data was received from New Horizons from its close encounter with Pluto on July 14,2015. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranuss orbit was being disturbed by another planet besides Neptune, by 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7,1915, but they were not recognized for what they were. There are fourteen other known prediscovery observations, with the oldest made by the Yerkes Observatory on August 20,1909. Percivals widow, Constance Lowell, entered into a legal battle with the Lowell Observatory over her late husbands legacy. Tombaughs task was to image the night sky in pairs of photographs, then examine each pair

2.
Voyager program
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The Voyager program is a continuing American scientific program that employs two robotic probes, Voyager 1 and Voyager 2, to study the outer Solar System. They were launched in 1977 to take advantage of an alignment of Jupiter, Saturn, Uranus, and Neptune. Their mission has been extended three times, and both continue to collect and relay useful scientific data. Neither Uranus nor Neptune has been visited by any other than Voyager 2. On August 25,2012, data from Voyager 1 indicated that it had become the first human-made object to enter space, traveling further than anyone, or anything. As of 2013, Voyager 1 was moving with a velocity of 17 kilometers per second relative to the Sun, Data and photographs collected by the Voyagers cameras, magnetometers, and other instruments revealed previously unknown details about each of the giant planets and their moons. Close-up images from the spacecraft charted Jupiter’s complex cloud forms, winds, saturn’s rings were found to have enigmatic braids, kinks, and spokes and to be accompanied by myriad ringlets. At Uranus Voyager 2 discovered a magnetic field around the planet and 10 additional moons. Its flyby of Neptune uncovered three complete rings and six hitherto unknown moons as well as a magnetic field and complex. Voyager 2 is still the only spacecraft to have visited the ice giants, the two Voyager space probes were originally conceived as part of the Mariner program, and they were thus initially named Mariner 11 and Mariner 12. The Voyager Program was similar to the Planetary Grand Tour planned during the late 1960s, the Grand Tour would take advantage of an alignment of the outer planets discovered by Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory. This alignment, which occurs once every 175 years, would occur in the late 1970s and make it possible to use gravitational assists to explore Jupiter, Saturn, Uranus, Neptune, and Pluto. The Planetary Grand Tour was to several pairs of probes to fly by all the outer planets along various trajectories, including Jupiter-Saturn-Pluto. Limited funding ended the Grand Tour program, but elements were incorporated into the Voyager Program, Voyager 2 was the first to launch. Its trajectory was designed to allow flybys of Jupiter, Saturn, Uranus and this encounter sent Voyager 1 out of the plane of the ecliptic, ending its planetary science mission. Had Voyager 1 been unable to perform the Titan flyby, the trajectory of Voyager 2 could have altered to explore Titan, forgoing any visit to Uranus. Voyager 1 was not launched on a trajectory that would have allowed it to continue to Uranus and Neptune, but could have continued from Saturn to Pluto without exploring Titan. The New Horizons probe, which had a higher velocity than Voyager 1, is traveling more slowly due to the extra speed Voyager 1 gained from its flybys of Jupiter

3.
Voyager 1
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Voyager 1 is a space probe launched by NASA on September 5,1977. Part of the Voyager program to study the outer Solar System, Voyager 1 launched 16 days after its twin, having operated for 39 years,7 months and 1 day, the spacecraft still communicates with the Deep Space Network to receive routine commands and return data. At a distance of 138 AU from the Sun as of March 2017, the probes primary mission objectives included flybys of Jupiter, Saturn, and Saturns large moon, Titan. It studied the weather, magnetic fields, and rings of the two planets and was the first probe to provide detailed images of their moons. After completing its mission with the flyby of Saturn on November 20,1980, Voyager 1 began an extended mission to explore the regions. On August 25,2012, Voyager 1 crossed the heliopause to become the first spacecraft to enter interstellar space, in the 1960s, a Grand Tour to study the outer planets was proposed which prompted NASA to begin work on a mission in the early 1970s. Information gathered by the Pioneer 10 spacecraft helped Voyagers engineers design Voyager to cope effectively with the intense radiation environment around Jupiter. Initially, Voyager 1 was planned as Mariner 11 of the Mariner program, due to budget cuts, the mission was scaled back to be a flyby of Jupiter and Saturn and renamed the Mariner Jupiter-Saturn probes. As the program progressed, the name was changed to Voyager. Voyager 1 was constructed by the Jet Propulsion Laboratory and it has 16 hydrazine thrusters, three-axis stabilization gyroscopes, and referencing instruments to keep the probes radio antenna pointed toward Earth. Collectively, these instruments are part of the Attitude and Articulation Control Subsystem, the spacecraft also included 11 scientific instruments to study celestial objects such as planets as it travels through space. The radio communication system of Voyager 1 was designed to be used up to, the communication system includes a 3. 7-meter diameter parabolic dish high-gain antenna to send and receive radio waves via the three Deep Space Network stations on the Earth. The craft normally transmits data to Earth over Deep Space Network Channel 18, using a frequency of either 2.3 GHz or 8.4 GHz, while signals from Earth to Voyager are broadcast at 2.1 GHz. When Voyager 1 is unable to communicate directly with the Earth, signals from Voyager 1 take over 19 hours to reach Earth. Voyager 1 has three radioisotope thermoelectric generators mounted on a boom, each MHW-RTG contains 24 pressed plutonium-238 oxide spheres. The RTGs generated about 470 W of electric power at the time of launch, the power output of the RTGs declines over time, but the crafts RTGs will continue to support some of its operations until 2025. As of 2017-04-06, Voyager 1 has 73. 14% of the plutonium-238 that it had at launch, by 2050, it will have 56. 5% left. Since the 1990s, space probes usually have completely autonomous cameras, the computer command subsystem controls the cameras

4.
Titan (moon)
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Titan is the largest moon of Saturn. It is the only known to have a dense atmosphere. Titan is the sixth ellipsoidal moon from Saturn, frequently described as a planet-like moon, Titan is 50% larger than Earths Moon, and it is 80% more massive. It is the second-largest moon in the Solar System, after Jupiters moon Ganymede, and is larger than the smallest planet, Mercury, discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite. Titan orbits Saturn at 20 Saturn radii, from Titans surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth. Titan is primarily composed of ice and rocky material. The geologically young surface is smooth, with few impact craters, although mountains. The atmosphere of Titan is largely nitrogen, minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas, and deltas, Huygens was inspired by Galileos discovery of Jupiters four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his brother Constantijn Huygens, Jr. began building telescopes around 1650 and it was the sixth moon to be discovered. He named it Saturni Luna, publishing in the 1655 tract De Saturni Luna Observatio Nova, after Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V. Other early epithets for Titan include Saturns ordinary satellite, Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion. Numerous small moons have been discovered closer to Saturn since then and he suggested the names of the mythological Titans, brothers and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, Titan orbits Saturn once every 15 days and 22 hours. Because of this, there is a point on its surface. Longitudes on Titan are measured westward, starting from the passing through this point. Its orbital eccentricity is 0.0288, and the plane is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, Titan reaches a distance of about 20 Saturn radii from Saturn

5.
Neptune
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Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the planet. Neptune is 17 times the mass of Earth and is more massive than its near-twin Uranus. Neptune orbits the Sun once every 164.8 years at a distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol ♆, Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the remaining known 14 moons were located telescopically until the 20th century. The planets distance from Earth gives it a small apparent size. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989, the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptunes composition can be compared and contrasted with the Solar Systems other giant planets, however, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasise this distinction. Traces of methane in the outermost regions in part account for the blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active, for example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the centre are approximately 5,400 K. Neptune has a faint and fragmented ring system. On both occasions, Galileo seems to have mistaken Neptune for a star when it appeared close—in conjunction—to Jupiter in the night sky, hence. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day and this apparent backward motion is created when Earths orbit takes it past an outer planet. Because Neptune was only beginning its yearly cycle, the motion of the planet was far too slight to be detected with Galileos small telescope

6.
Triton (moon)
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Triton is the largest natural satellite of the planet Neptune. It was discovered on October 10,1846, by English astronomer William Lassell and it is the only large moon in the Solar System with a retrograde orbit, an orbit in the opposite direction to its planets rotation. At 2,700 kilometres in diameter, it is the seventh-largest moon in the Solar System, because of its retrograde orbit and composition similar to Plutos, Triton is thought to have been a dwarf planet captured from the Kuiper belt. Triton has a surface of frozen nitrogen, a mostly water-ice crust, an icy mantle. The core makes up two-thirds of its total mass, Triton has a mean density of 2.061 g/cm3 and is composed of approximately 15–35% water ice. Triton is one of the few moons in the Solar System known to be geologically active, as a consequence, its surface is relatively young with sparse impact craters, and a complex geological history revealed in intricate cryovolcanic and tectonic terrains. Part of its surface has geysers erupting sublimated nitrogen gas, contributing to a nitrogen atmosphere less than 1/70,000 the pressure of Earths atmosphere at sea level. Triton was discovered by British astronomer William Lassell on October 10,1846, a brewer by trade, Lassell began making mirrors for his amateur telescope in 1820. When John Herschel received news of Neptunes discovery, he wrote to Lassell suggesting he search for possible moons, Lassell did so and discovered Triton eight days later. Lassell also claimed to have discovered rings, although Neptune was later confirmed to have rings, they are so faint and dark that it is doubtful that he actually saw them. Triton is named after the Greek sea god Triton, the son of Poseidon, the name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire, and was officially adopted many decades later. Until the discovery of the second moon Nereid in 1949, Triton was commonly referred to as the satellite of Neptune. Lassell did not name his own discovery, he successfully suggested the name Hyperion, previously chosen by John Herschel. Triton is unique among all large moons in the Solar System for its orbit around its planet. Most of the irregular moons of Jupiter and Saturn also have retrograde orbits. However, these moons are all much more distant from their primaries, and are small in comparison, Tritons orbit is associated with two tilts, the inclination of Neptunes spin to Neptunes orbit, 30°, and the inclination of Tritons orbit to Neptunes spin, 157°. Tritons orbit precesses forward relative to Neptunes spin with a period of about 678 Earth years and that inclination is currently 130°, Tritons orbit is now near its maximum departure from coplanarity with Neptunes. Tritons rotation is locked to be synchronous with its orbit around Neptune

7.
Jupiter
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Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, Jupiter and Saturn are gas giants, the other two giant planets, Uranus and Neptune are ice giants. Jupiter has been known to astronomers since antiquity, the Romans named it after their god Jupiter. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium and it may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rotation, the planets shape is that of an oblate spheroid. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence, a prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere, Jupiter has at least 67 moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. In late February 2007, Jupiter was visited by the New Horizons probe, the latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4,2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa, Earth and its neighbor planets may have formed from fragments of planets after collisions with Jupiter destroyed those super-Earths near the Sun. Astronomers have discovered nearly 500 planetary systems with multiple planets, Jupiter moving out of the inner Solar System would have allowed the formation of inner planets, including Earth. Jupiter is composed primarily of gaseous and liquid matter and it is the largest of the four giant planets in the Solar System and hence its largest planet. It has a diameter of 142,984 km at its equator, the average density of Jupiter,1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets. Jupiters upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, a helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiters atmosphere is approximately 75% hydrogen and 24% helium by mass, the atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, the outermost layer of the atmosphere contains crystals of frozen ammonia. The interior contains denser materials - by mass it is roughly 71% hydrogen, 24% helium, through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found

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Saturn
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Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with a radius about nine times that of Earth. Although it has only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive, Saturn is named after the Roman god of agriculture, its astronomical symbol represents the gods sickle. Saturns interior is composed of a core of iron–nickel and rock. This core is surrounded by a layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium. Saturn has a yellow hue due to ammonia crystals in its upper atmosphere. Saturns magnetic field strength is around one-twentieth of Jupiters, the outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, higher than on Jupiter, sixty-two moons are known to orbit Saturn, of which fifty-three are officially named. This does not include the hundreds of moonlets comprising the rings, Saturn is a gas giant because it is predominantly composed of hydrogen and helium. It lacks a definite surface, though it may have a solid core, Saturns rotation causes it to have the shape of an oblate spheroid, that is, it is flattened at the poles and bulges at its equator. Its equatorial and polar radii differ by almost 10%,60,268 km versus 54,364 km, Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. Saturn is the planet of the Solar System that is less dense than water—about 30% less. Although Saturns core is considerably denser than water, the specific density of the planet is 0.69 g/cm3 due to the atmosphere. Jupiter has 318 times the Earths mass, while Saturn is 95 times the mass of the Earth, together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. On 8 January 2015, NASA reported determining the center of the planet Saturn, the temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to transition into a metal in the deeper layers. Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a rocky core surrounded by hydrogen. This core is similar in composition to the Earth, but more dense, in 2004, they estimated that the core must be 9–22 times the mass of the Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude

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Uranus
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Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System, Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as ice giants to distinguish them from the gas giants, the interior of Uranus is mainly composed of ices and rock. Uranus is the planet whose name is derived from a figure from Greek mythology. Like the other giant planets, Uranus has a system, a magnetosphere. The Uranian system has a unique configuration among those of the planets because its axis of rotation is tilted sideways and its north and south poles, therefore, lie where most other planets have their equators. In 1986, images from Voyager 2 showed Uranus as an almost featureless planet in visible light, observations from Earth have shown seasonal change and increased weather activity as Uranus approached its equinox in 2007. Wind speeds can reach 250 metres per second, like the classical planets, Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit. Uranus had been observed on many occasions before its recognition as a planet, possibly the earliest known observation was by Hipparchos, who in 128 BCE might have recorded it as a star for his star catalogue that was later incorporated into Ptolemys Almagest. The earliest definite sighting was in 1690 when John Flamsteed observed it at least six times, the French astronomer Pierre Lemonnier observed Uranus at least twelve times between 1750 and 1769, including on four consecutive nights. Sir William Herschel observed Uranus on March 13,1781 from the garden of his house at 19 New King Street in Bath, Somerset, England, Herschel engaged in a series of observations on the parallax of the fixed stars, using a telescope of his own design. Herschel recorded in his journal, In the quartile near ζ Tauri, either Nebulous star or perhaps a comet. On March 17 he noted, I looked for the Comet or Nebulous Star and found that it is a Comet, the sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed. Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23,1781, I dont know what to call it. It is as likely to be a planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it, although Herschel continued to describe his new object as a comet, other astronomers had already begun to suspect otherwise. Finnish-Swedish astronomer Anders Johan Lexell, working in Russia, was the first to compute the orbit of the new object and its nearly circular orbit led him to a conclusion that it was a planet rather than a comet. Berlin astronomer Johann Elert Bode described Herschels discovery as a star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn

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Jet Propulsion Laboratory
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The Jet Propulsion Laboratory is a federally funded research and development center and NASA field center in La Cañada Flintridge, California and Pasadena, California, United States. The JPL is managed by the nearby California Institute of Technology for NASA, the laboratorys primary function is the construction and operation of planetary robotic spacecraft, though it also conducts Earth-orbit and astronomy missions. It is also responsible for operating NASAs Deep Space Network and they are also responsible for managing the JPL Small-Body Database, and provides physical data and lists of publications for all known small Solar System bodies. The JPLs Space Flight Operations Facility and Twenty-Five-Foot Space Simulator are designated National Historic Landmarks, JPL traces its beginnings to 1936 in the Guggenheim Aeronautical Laboratory at the California Institute of Technology when the first set of rocket experiments were carried out in the Arroyo Seco. Malinas thesis advisor was engineer/aerodynamicist Theodore von Kármán, who arranged for U. S. Army financial support for this GALCIT Rocket Project in 1939. In 1941, Malina, Parsons, Forman, Martin Summerfield, in 1943, von Kármán, Malina, Parsons, and Forman established the Aerojet Corporation to manufacture JATO motors. The project took on the name Jet Propulsion Laboratory in November 1943, during JPLs Army years, the laboratory developed two deployed weapon systems, the MGM-5 Corporal and MGM-29 Sergeant intermediate range ballistic missiles. These missiles were the first US ballistic missiles developed at JPL and it also developed a number of other weapons system prototypes, such as the Loki anti-aircraft missile system, and the forerunner of the Aerobee sounding rocket. At various times, it carried out testing at the White Sands Proving Ground, Edwards Air Force Base. A lunar lander was developed in 1938-39 which influenced design of the Apollo Lunar Module in the 1960s. The team lost that proposal to Project Vanguard, and instead embarked on a project to demonstrate ablative re-entry technology using a Jupiter-C rocket. They carried out three successful flights in 1956 and 1957. Using a spare Juno I, the two organizations then launched the United States first satellite, Explorer 1, on February 1,1958, JPL was transferred to NASA in December 1958, becoming the agencys primary planetary spacecraft center. JPL engineers designed and operated Ranger and Surveyor missions to the Moon that prepared the way for Apollo, JPL also led the way in interplanetary exploration with the Mariner missions to Venus, Mars, and Mercury. In 1998, JPL opened the Near-Earth Object Program Office for NASA, as of 2013, it has found 95% of asteroids that are a kilometer or more in diameter that cross Earths orbit. JPL was early to employ women mathematicians, in the 1940s and 1950s, using mechanical calculators, women in an all-female computations group performed trajectory calculations. In 1961, JPL hired Dana Ulery as their first woman engineer to work alongside male engineers as part of the Ranger and Mariner mission tracking teams, when founded, JPLs site was a rocky flood-plain just outside the city limits of Pasadena. Almost all of the 177 acres of the U. S, the city of La Cañada Flintridge, California was incorporated in 1976, well after JPL attained international recognition with a Pasadena address

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Discovery Program
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NASAs Discovery Program is a series of lower-cost, highly focused American scientific space missions that are exploring the Solar System. It was founded in 1992 to implement then-NASA Administrator Daniel S. Goldins vision of faster, better, cheaper planetary missions, Discovery missions differ from traditional NASA missions where targets and objectives are pre-specified. Instead, these missions are proposed and led by a scientist called the Principal Investigator. Proposing teams may include people from industry, small businesses, government laboratories, proposals are selected through a competitive peer review process. All of the completed Discovery missions are accomplishing ground-breaking science and adding significantly to the body of knowledge about the Solar System, NASA also accepts proposals for competitively selected Discovery Program Missions of Opportunity. These opportunities are offered through NASAs Stand Alone Mission of Opportunity program. In 1989, the Solar System Exploration Division at NASA Headquarters initiated a series of workshops to define a new strategy for exploration through the year 2000, a fast-paced study for a potential mission was requested and funding arrangements were made in 1990. On February 17,1996, NEAR became the first mission to launch in the Discovery Program, also, Phoenix and MAVEN were in the Mars Scout Program not Discovery program. NEAR Shoemaker, a mission to study asteroid 433 Eros, launched on February 17,1996, the spacecraft entered orbit around Eros in 2000 and successfully touched down on its surface one year later. It has succeeded its primary and extended mission and is now complete, the Project Scientist was Andrew Chang of The Johns Hopkins University Applied Physics Laboratory. Mars Pathfinder, a Mars lander to deploy Sojourner rover on the surface, launched in 1996, it landed on Mars on July 4,1997. It has completed its primary and extended mission, the Principal Investigator was Matthew Golombek of NASA’s Jet Propulsion Laboratory. Lunar Prospector, a Moon orbiter to characterize the lunar mineralogy, launched in 1998, it spent 1½ years in lunar orbit. It has completed its primary and extended mission and deliberately impacted onto the Moons surface, the Principal Investigator was Alan Binder of the Lunar Research Institute. Stardust, a mission to collect interstellar dust and dust particles from the nucleus of comet 81P/Wild for study on Earth, launched in 1999, it successfully collected samples between 2000–2004, then the sample return capsule returned to Earth on January 15,2006. The capsule is on display at the National Air and Space Museum in Washington D. C, scientists worldwide are studying the comet dust samples while citizen scientists are finding interstellar dust bits through the Stardust@home project. The spacecraft was assigned a new task, called Stardust-NExT to revisit Tempel 1, having completed its primary and extend missions, Stardust did a final burn to deplete its remaining fuel on March 24,2011. The Principal Investigator was Donald Brownlee of the University of Washington, Genesis, a mission to collect solar wind charged particles for analysis on Earth

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New Frontiers program
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The New Frontiers program is a series of space exploration missions being conducted by NASA with the purpose of researching several of the Solar System bodies, including the dwarf planet Pluto. NASA is encouraging both domestic and international scientists to submit proposals for the project. New Frontiers was built on the approach used by the Discovery. It is designed for missions that cannot be accomplished within the cost and time constraints of Discovery. The New Frontiers program was developed and advocated by NASA and granted by Congress in CY2002 and 2003 and this effort was led by two long-time NASA executives at Headquarters at that time, Edward Weiler, Associate Administrator of Science and Colleen Hartman, Solar System Exploration Division Director. The 2003 Planetary Science Decadal Survey from the National Academy of Sciences identified destinations that then served as the source of the first competition for the New Frontiers program. The program name was selected by Hartman based on President John F. Kennedys speech in 1960, in which he said We stand, today, examples of proposed mission concepts include two tranches of several mission concepts based on decadal survey goals. After a Jupiter gravity assist in February 2007 the spacecraft continued towards Pluto, the primary mission flyby occurred in July 2015 and the spacecraft was then targeted toward one Kuiper Belt object called 2014 MU69 for a January 1,2019 flyby. Another mission that was considered with this mission was New Horizons 2, juno is a Jupiter exploration mission which launched on August 5,2011 and arrived in July 2016. It is the first solar-powered spacecraft to explore an outer planet, the craft was placed into a polar orbit in order to study the planets magnetic field and internal structure. OSIRIS-REx stands for Origins, Spectral Interpretation, Resource Identification, Security and this mission plan is to orbit an asteroid, at the time named 1999 RQ36, by 2020. After extensive measurements, the spacecraft will collect a sample from the surface for return to Earth in 2023. The mission, excluding the launch vehicle, is expected to cost approximately $800 million, the returned sample will help scientists answer long-held questions about the formation of the Solar System and the origin of complex organic molecules necessary for the origin of life. Asteroid Bennu is a potential future Earth impactor and is listed on the Sentry Risk Table with the third highest rating on the Palermo Technical Impact Hazard Scale. In the late 2100s there is a chance of about 0. 07% it could strike Earth, therefore there is a need to measure the composition. Investigators may propose the use of Multi-Mission Radioisotope Thermoelectric Generators, the Decadal Survey additionally recommended the Io Observer and Lunar Geophysical Network proposals for New Frontiers 5 in additional to the previous recommendations. NASAs Planetary Science Division responded to the Decadal Survey with support, cosmic Vision New Frontiers Program website

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass …

Jupiter's diameter is one order of magnitude smaller (×0.10045) than that of the Sun, and one order of magnitude larger (×10.9733) than that of Earth. The Great Red Spot is roughly the same size as Earth.